In order to reduce the molding cycle of injection molding machines, such machines have been provided with a valve adapted to cut off communication between a mold and a plasticizer (i.e., extruder) as soon as the plasticizer has completed injecting plastic material into the mold, whereby the plasticizer can start to accumulate plastic material for another shot as the plastic material in the mold cools (see, for instance, U.S. Pat. No. Re. 28,721). An auxiliary piston and cylinder arrangement performs a packing operation by maintaining pressure on the plastic material in the mold and supplying additional plastic material to the mold in order to compensate for shrinkage during the cooling cycle. At the conclusion of the cooling cycle, the auxiliary piston and cylinder arrangement also performs a suck back operation by which plastic material contained in a runner is severed from the plastic material in the mold.
In order to further reduce the molding cycle of injection molding machines, such machines have utilized two plasticizers which function alternately (see, for instance, U.S. Pat. Nos. 3,940,223 and 4,070,142). The two plasticizers operate in synchronization such that one is prepared for the next injection operation, while the other one is injecting plastic material into the mold. More particularly, as soon as either plasticizer has injected plastic material into the mold, a valve shuts off that plasticizer from communication with the mold. A piston and cylinder arrangement is then employed to perform the packing and the suck back operations described above. Thus, the piston and cylinder arrangement performs two operations which would normally be performed by the plasticizers. However, because the plasticizers still perform a mold filling function, the molding machines which utilize such plasticizers still suffer from two disadvantages which are common to screw type plasticizers: namely, relatively low injection pressures and slow fill rates.